The price war between Coles and Woolworths is proving a boon to motorists accustomed to paying premium prices when they fill up the tank at the weekend.

Coles is offering petrol discounts of between 12¢ and 18¢ a litre to FlyBuys loyalty card members who spend at least $50 and $130 on groceries between Friday and Sunday this week.

Last weekend, Woolworths offered its most loyal customers a petrol discount of 30¢ a litre when they spent at least $200 in supermarkets. Woolworths plans to offer a different group of Everyday Rewards customers the same 30¢ a litre offer this weekend.

Petrol has become the latest weapon in the battle for market share between Woolworths and Coles.

In October, Coles doubled its standard 4¢ a litre discount to 8¢ a litre until Easter for customers who spend at least $30.

Woolworths followed suit in mid-January, as holiday makers returned to work, after seeing petrol volumes fall in the December quarter.

Woolworths is also doubling reward points this weekend, offering two Everyday Rewards points for every $1 spent over $30 for Everyday Rewards members who have linked their cards to the Qantas Frequent Flyer program.

However, as the cost of loyalty rises, the retailers are making more targeted offers to customers to increase the effectiveness of the promotions.